Optical storage media are media in which data are stored in an optically readable manner, for example by means of a laser and a photo-detector being integrated within a pickup. The photo-detector is used for detecting the reflected light of the laser beam when reading data from the storage medium. In the meanwhile a large variety of optical storage media are known, which are operated with different laser wavelength, and which have different sizes for providing storage capacities from below one Gigabyte up to 50 Gigabyte (GB). The formats include read-only formats such as Audio CD and Video DVD, write-once optical media such as CD-R and DVD-R, DVD+R, as well as rewritable formats like CD-RW, DVD-RW and DVD+RW. Digital data are stored on these media along tracks in one or more layers of the media.
The storage medium with the highest data capacity is at present the Blu-Ray disc (BD), which allows to store 50 GB on a dual layer disc. Available Blu-Ray formats are at present read only BD-ROM, re-writable BD-RE and write once BD-R discs. For reading and writing of a Blu-Ray disc an optical pickup with a laser wavelength of 405 nm is used. On the Blu-Ray disc a track pitch of 320 nm and a mark length from 2T to 8T and 9T is used, where T is the channel bit length, and which corresponds with a minimum mark length of 138-160 nm. The re-writable BD-RE disc is based on a phase change technology comprising a phase change layer, which uses for example a compound of AgInSbTe or GeSbTe. Further information about the Blu-Ray disc system is available for example from the Blu-Ray group via internet: www.blu-raydisc.com.
New optical storage media with a super resolution near-field structure (Super-RENS) offer the possibility to increase the data density of the optical storage medium by a factor of three to four in one dimension as compared with the Blu-Ray disc. This is possible by a so-called Super-RENS structure, which is placed above a data layer of the optical storage medium, and which significantly reduces the effective size of a light spot used for reading from or writing to the optical storage medium. The super resolution layer is also called a mask layer because it is arranged above the data layer and for some materials only the high intensity center part of a laser beam can penetrate the mask layer. Further, materials can be used for the mask layer which show a higher reflectivity in the center part of the focused laser beam, e.g. InSb shows this nonlinear optical property.
The Super-RENS effect allows to record and read data stored in marks of an optical disc, which have a size below the resolution limit of an optical pick-up used for reading or writing the data on the disc. As known, the optical diffraction limit of the resolution of an optical pick-up is about lambda/(2 NA), where lambda is the laser wavelength and NA the numerical aperture of the objective lens of the optical pickup.
A super resolution near field technique for recording and retrieving small marks beyond the optical diffraction limit is described be Tominaga, Nakano and Atoda in “An approach for recording and readout beyond the diffraction limit with an Sb thin film”, Applied Physics Letters, Vol. 73, No. 15, 12 Oct. 1998, which describe to use an Sb thin film as the super resolution layer.
It is known that also semiconductor materials can be used as a mask layer for Super-RENS optical storage media, for example ZnO. A semiconductor material of this kind for a Super-RENS layer is described by Takamori et al, “Energy-Gap-Induced super-Resolution Optical Disc using ZnO Interference Film”, Japanese Journal of Applied Physics, Vol. 44, No. 5b, 2005, pp. 3627-3630. Takamori et al describe a Super-RENS disc with ZnO as an active layer deposited on a ROM type substrate and show that a temperature rise can locally increase the ZnO transmittance, thus triggering a near field interaction capable of below-diffraction-limit detection.
In the articles Hyot et al, “Phase change materials and Super-RENS”, E*PCOS 05, Technical Digest, Cambridge, 2005, and Pichon et al, “Multiphysics Simulation of Super-Resolution BD ROM Optical Disk Readout” 2006 IEEE, 0-7803-9494-1/06, PP 206-208, a semi-conducting mask layer is proposed in which a local change of the refractive index can be obtained through photo generation of free carriers. A thermal description is given to provide information on temperature distribution during readout of the data layer.
In US2003/0193857 an optical disc comprising a Super-RENS mask layer is described comprising a semiconductor film which can have a contamination or a matrix material mixed into the semiconductor. The Super-RENS detection is based on an increase of the transmittance of the mask layer, with the transmittance being increased by absorption saturation of the semiconductor layer upon radiation with an incident laser beam. The mask layer may include impurities, which allows to shift the energy gap such that efficient absorption is obtained for a certain wavelength.
The Super-RENS layers at present under development for future optical storage media have the drawback that a comparatively high laser power is needed to activate the mask layer by high temperature or high laser power. There is also a need to provide a Super-RENS recordable disc having a high long-time stability.